Our invention is adapted to be used in automatic transmission for automotive vehicles employing a hydrokinetic torque converter and multiple ratio gearing. Such converter and gearing arrangements may be disposed in either an inline arrangement wherein the output shaft is coaxially disposed with respect to the axis of the converter and with respect to the crankshaft of the engine or in a transaxle arrangement wherein the engine and converter are located on one axis and the output elements of the gearing are mounted for rotation about a transverse axis. In either case engine torque is delivered to the impeller of the converter and the turbine of the converter acts as a driver for the torque input elements of the gearing. Fluid pressure operated brakes are employed for establishing reaction points for the gearing and multiple fluid operated clutches are employed for establishing a connection between the turbine and selected torque input elements of the gearing.
Such control systems employ a fluid pressure pump driven by the engine and a control circuit that responds to engine torque and vehicle speed to effect engagement and disengagement of the clutches and brakes. It is usual practice for the turbine to remain connected to the torque input element that establishes a low speed ratio condition in the gearing when the vehicle is brought to a stop by applying the vehicle brakes although the magnitude of the torque delivered through the converter is low when the vehicle is stopped and the engine is idling, a minimal torque delivery to the traction wheels nevertheless is established which tends to cause the vehicle to creep. The engine, furthermore, must overcome the load imposed on it because of this residual torque transfer; and this is done at the expense of engine fuel economy and the quality of engine exhaust gas emissions.
The improvement of our invention makes it possible to disconnect the turbine from the torque input elements of the gearing when the engine is idling and the vehicle is stopped or when the vehicle is braked while the vehicle is traveling at a speed less than the selected minimum value. This is accomplished by the use of valve components that disengage the friction clutch between the turbine and the torque input element of the gearing but that allows for a threshold pressure to remain in the servo for the clutch so that a subsequent engagement of the clutch will occur instantaneously upon depression of the engine carburetor throttle during vehicle startup.
We are aware of various prior art teachings that relate to the general concept of establishing a neutral-idle condition in a vehicle driveline. Examples of these are shown in U.S. Pat. No. 2,608,880 (Flinn) wherein a transmission control functions to interrupt torque delivery through a powertrain when the vehicle brakes are applied and the speed of the transmission tailshaft is below a certain value. Provision is made by Flinn for maintaining the powertrain in a torque delivery condition even though the accelerator is returned to its closed position without reference to application of the brakes when the speed is above the designed value.
Another example of prior art having a neutral-idle condition is the circuit shown in U.S. Pat. No. 4,298,109 (Dorpmund et al). That reference shows a main control valve between an automatic transmission low speed ratio clutch which responds to governor pressure and to a torque dependent pressure to deliver control pressure directly to the clutch when the governor pressure and the torque dependent pressure are sufficient to overcome the force of the valve spring. When the brakes are applied, however, a solenoid valve is actuated thereby eliminating the influence of the torque dependent pressure. The control valve responds to that loss of torque dependent pressure by exhausting of the pressure in the clutch as the spring overcomes the force of the remaining governor pressure. The control valve spring produces a predominant valve force only if both of the opposing control pressures are available to the control valve. Speed sensitive pressure alone is sufficient to overcome the spring force when the vehicle speed exceeds a predetermined value. Whenever the spring overcomes the combined forces of the speed and torque signals, the clutch is opened to exhaust and thus disengages.
Another example of a neutral-idle control system is shown in U.S. Pat. No. 4,105,101, which includes a regulator valve for controlling the application of a clutch or brake used to establish a low speed ratio drive. The valve is controlled by a solenoid valve that responds to application of the vehicle brakes and accelerator pedal displacement to open an exhaust flow path for the clutch or brake, the exhaust flow path including a calibrated flow restricting orifice between the control valve and the solenoid valve. The regulator valve for the brake that establishes a low speed ratio condition maintains a residual pressure in the brake. A brake switch and a speed sensitive switch arranged in series causes the solenoid valve to be actuated when the brakes are applied and the engine throttle is closed. The solenoid valve will remain energized even after the brakes are released if the speed is lower than the design limit or if the vehicle is stopped and the engine throttle remains closed. A holding circuit achieves this function.
U.S. Pat. No. 4,331,045 shows still another neutral-idle control which comprises a speed governor sensitive control valve (main control valve) and an accelerator controlled purging valve for controlling the effect of a load dependent pressure on the control valve.